Today's semiconductor manufacturing industry is rapidly developing under the guidance of Moore's Law, so as to continuously improve performance and integration density of integrated circuits while reducing power consumption as much as possible. Preparation of a high-performance, low-power ultra short channel device becomes the focus of the future semiconductor manufacturing industry. After entering the 22 nm technology node, in a conventional planar field effect transistor a leakage current is increasing due to a growing short channel effect, which can not meet the development of semiconductor manufacturing. In order to overcome the above issues, a multi-gate structure device is gradually attracting widespread attentions, because it can increase the driving current density per unit area while overcoming the short channel effect due to excellent gate control properties and transport characteristics thereof.
Although the multi-gate structure device itself has a characteristic geometry structure, so that it has excellent gate control performance, this will make a body effect factor decrease. Accordingly, in comparison to a conventional planar device, it is difficult to achieve a multi-threshold device from a multi-gate structure device. Under the same process conditions, a conventional method of achieving a multi-threshold device is to conduct a channel doping, and a multi-gate structure device requires a higher channel doping concentration in order to achieve a threshold voltage variation. However, due to the presence of Coulomb impurity scattering, improvement of a channel doping concentration will seriously affect the mobility of carriers in the device, finally so that the driving current is dropped significantly. This is one of the major difficulties and challenges when a multi-gate structure device is applied to a large scale integrated circuit product.